Investigating the coronal structure by studying time lags in the Atoll source 4U 1705-44 using AstroSat

TL;DR

This study uses AstroSat timing data to analyze time lags in the Atoll source 4U 1705-44, revealing insights into the accretion disk and corona structure, including lag times and coronal height estimations.

Contribution

It provides the first detailed timing analysis of 4U 1705-44 using AstroSat, identifying lag times and constraining the coronal and boundary layer geometry around the neutron star.

Findings

Detection of soft and hard lags of tens to hundreds of seconds.

Identification of a broad noise component at ~13 Hz in the 3-10 keV band.

Estimated coronal height of around 15 km.

Abstract

We performed a detailed timing study of the Atoll source 4U 1705-44 in order to understand the accretion disk geometry. Cross correlation function (CCF) studies were performed between soft (3-5 keV) and hard energy (15-30 keV) bands using the AstroSat LAXPC data. We detected hard as well as soft lags of the order of few ten to hundred seconds. A dynamical CCF study was performed in the same energy bands for one of the light curves and we found smaller lags of few tens of seconds ($<$ 50 s) suggesting that the variation is probably originating from the corona. We found a broad noise component around $\sim$ 13 Hz in the 3-10 keV band which is absent in 10-20 keV band. We interpret the observed lags as the readjustment timescales of the corona or a boundary layer around the neutron star and constrain the height of this structure to few tens of km. We independently estimated the coronal height to be around 15 km assuming that the 13 Hz feature in the PDS is originating from the oscillation of the viscous shell around the neutron star.